In many applications, it is desired to obtain a direct-current (DC) power from alternating-current (AC) line. Usually, the DC output voltage is low compared to the AC line voltage. For example, in a household electronic appliance, an AC line voltage having a magnitude of 220 volts and a frequency of 50 HZ is converted into a DC power having an output voltage of 5 volts and an output current of 30 mA to supply a microprocessor inside the appliance. Switching converter, such as high-side buck converter, is commonly used to obtain the desired DC power from AC line, referring to FIG. 1. Magnetic inductor Li and high voltage capacitors C1 and C2 are necessary to obtain the desired power at node OUT, which leads to bulky size and high bill-of-material (BOM) cost. Furthermore, the high frequency switching of switch M may result in high Electro-Magnetic-Interference (EMI) noise.
Accordingly, an improved method is desired to overcome at least part of the above mentioned deficiencies.